Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
  • H04B1/69—Spread spectrum techniques
  • H04B1/7163—Spread spectrum techniques using impulse radio
  • H04B1/7176—Data mapping, e.g. modulation
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/18—Phase-modulated carrier systems, i.e. using phase-shift keying
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/18—Phase-modulated carrier systems, i.e. using phase-shift keying
  • H04L27/20—Modulator circuits; Transmitter circuits
  • H04L27/2032—Modulator circuits; Transmitter circuits for discrete phase modulation, e.g. in which the phase of the carrier is modulated in a nominally instantaneous manner
  • H04L27/2053—Modulator circuits; Transmitter circuits for discrete phase modulation, e.g. in which the phase of the carrier is modulated in a nominally instantaneous manner using more than one carrier, e.g. carriers with different phases
  • H04L27/206—Modulator circuits; Transmitter circuits for discrete phase modulation, e.g. in which the phase of the carrier is modulated in a nominally instantaneous manner using more than one carrier, e.g. carriers with different phases using a pair of orthogonal carriers, e.g. quadrature carriers
  • H04L27/2067—Modulator circuits; Transmitter circuits for discrete phase modulation, e.g. in which the phase of the carrier is modulated in a nominally instantaneous manner using more than one carrier, e.g. carriers with different phases using a pair of orthogonal carriers, e.g. quadrature carriers with more than two phase states
  • H04L27/2071—Modulator circuits; Transmitter circuits for discrete phase modulation, e.g. in which the phase of the carrier is modulated in a nominally instantaneous manner using more than one carrier, e.g. carriers with different phases using a pair of orthogonal carriers, e.g. quadrature carriers with more than two phase states in which the data are represented by the carrier phase, e.g. systems with differential coding
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/18—Phase-modulated carrier systems, i.e. using phase-shift keying
  • H04L27/22—Demodulator circuits; Receiver circuits
  • H04L27/233—Demodulator circuits; Receiver circuits using non-coherent demodulation
  • H04L27/2331—Demodulator circuits; Receiver circuits using non-coherent demodulation wherein the received signal is demodulated using one or more delayed versions of itself
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
  • H04B1/69—Spread spectrum techniques
  • H04B1/7163—Spread spectrum techniques using impulse radio
  • H04B1/71635—Transmitter aspects
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
  • H04B1/69—Spread spectrum techniques
  • H04B1/7163—Spread spectrum techniques using impulse radio
  • H04B1/719—Interference-related aspects
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
  • H04B1/69—Spread spectrum techniques
  • H04B2001/6908—Spread spectrum techniques using time hopping

Definitions

• the invention relates generally to communication systems, and more particularly to modulation formats used in wireless communication systems.
• UWB ultra-wide bandwidth
• the FCC order also limits the power spectral density and peak emissions power of UWB signals, e.g. less than -43.1 dBm/MHz.
• One modulation method for UWB uses extremely short time pulses to generate signals with bandwidths greater than 500 MHz, e.g., 1/1,000,000,000 of a second or less, which corresponds to a wavelength of about 300 mm.
• Systems that use short pulses are commonly referred to as impulse radio (IR) systems.
• PPM pulse position modulation
• PAM pulse amplitude modulation
• OOK on-off keying
• BPSK bi-phase shift keying
• UWB systems can achieve high data rates, and are resistant to multi-path impairments due to the large processing gains. Additionally, the use of IR based UWB technology allows for the implementation of low cost, low duty cycle, low power transceivers that do not require local oscillators for heterodyning. Because UWB radios are primarily digital circuits, they can easily be integrated in a semiconductor chip. In UWB systems, multiple users can simultaneously share the same spectrum with no interference to one another, and are ideal for high-speed home and business networking devices, as well as sensor networks.
• the IEEE 802.15.4a standard defines a physical-layer for communications with scalable data rates from 1 Kbs to 1 Mbps, "IEEE P802.15.4a WPAN Alternate PHY - PAR," 2003, for low power, low data rate network.
• IR systems are either time-hopped (TH-IR), or transmitted- reference (TR-IR). Both systems use sequences of short duration pulses, p ⁇ t).
• TH-IR time-hopped
• TR-IR transmitted- reference
• modulation and demodulation for TH-IR and TR-IR differ significantly, making TH-IR and TR-IR incompatible in the same network.
• TH-IR system are described by M. Win and R. A. Scholtz, "Ultra-Wide Band Width Time-Hopping Spread-Spectrum Impulse Radio for Wireless Multiple- Access Communications, " in IEEE Trans. On Communications, Vol. 48, No. 4 April 2000, pp. 679- 691.
• each bit or symbol is represented by Ny pulses, where N/ is a positive integer.
• the time taken to transmit the bit is T s . This is called the symbol duration.
• the time T s is further partitioned into frames Tf, and the frames are partitioned into chips T 0 corresponding typically to a pulse duration. If N c represents the number of chips in a frame and N/ represents the number of frames in a symbol, then T 3 , Tf and T c are related as follows
• Figure IB shows the relationship the symbol time T s 101, the frame time Tf
• the pulses are spaced pseudo-randomly among the available chips in a frame according to a
• time-hopping code to minimize the effect of multi user interference.
• each bit b is represented as either a positive or negative one b E ⁇ -1,1 ⁇ .
• the transmitted signal has the form
• c,- represents they ⁇ th ' value of the TH code, in the range ⁇ 0,1,... ,N C -1 ⁇ , and b is the /' modulation symbol.
• an optional sequence denoted as hj j can be applied to each pulse in the transmitted signal so as to shape the spectrum of the transmitted signal and to reduce spectral lines.
• the sequence, hy is called a polarity scrambling sequence with values of either +1 or -1. Different amplitudes are possible to give further degrees of freedom in the shaping of the spectrum.
• FIG. 2 shows a conventional coherent TH-IR receiver 200.
• the receiver includes an automatic gain control (AGC) unit 210 coupled to an amplifier 220 that is connected to the receive antenna 230.
• the receiver also includes synchronization 240, timing control 250, channel estimation 260, MMSE equalizer 270, and decoder 280 units.
• Rake receiver fingers 290 input to an adder 295.
• Each rake finger includes a pulse sequence generator, correlator and weight combiner. The rake fingers reduce multipath interference. Due to the density of the multipaths in UWB signals, the number of required RAKE fingers can be large to obtain reasonable performance.
• the output of the adder is equalized and decoded.
• the typical TH-IR receiver has a significant complexity.
• TR-IR systems eliminate the need for a RAKE receiver, R. Hoctor and H.
• the infonnation is encoded as phase differences of successive pulses in the sequence.
• Each symbol in a TR-IR system is a sequence of time-hopped 'doublets' or pair of two consecutive pulses.
• the first pulse in the pair is referred to as a reference pulse and the second pulse is referred to as a data pulse.
• the two pulses in each pair are separated by a fixed unit of time T d . Multiple pairs can be transmitted for one information bit.
• the transmitted waveform has the form
• Figure 3 shows the relationship the symbol time T s 301, the frame time 7 ⁇ 302, and the chip time T c 303 for pulses 304 for an example TH-IR waveform 310 for a ' 0 ' bit, and waveform 320 for a ' 1 ' bit.
• Figure 4 shows a conventional TR-IR receiver 400, which is significantly simpler than the TH-IR receiver of Figure 2.
• the receiver includes delay 401, multiplier 402, integrator 403, sampler 407 and decision 404 units.
• the receiver essentially correlates the received signal 405 with a delayed version 406.
• the TR-IR 400 receiver is less complex than the TH-IR receiver 200.
• the reduced complexity is at the cost of requiring twice the number of pulses, and the additional energy required for the reference pulses, nominally 3dB or more.
• the invention provides a system and method for incorporating TH-IR and TR-IR transceivers in the same wireless network.
• the invention also provides a modulation format that encodes information bits is such a way to enable both TH- IR and TR-IR receivers to demodulate the same signals.
• the modulation format does not suffer from the inherent 3dB loss when the TH-IR receiver is used.
• the invention can be applied to narrow band, wide band, and ultra-wide band radio systems.
• a method modulates a sequence of bits in a wireless communications network by generating a reference waveform, e.g., a pulse, and a data waveform, e.g., another pulse, of a waveform pair for each current bit.
• the phase of the reference waveform depends on a previously modulated bit, and a difference in phase (polarity) between the reference waveform and the data waveform pair depend on the current bit.
• a symbol period for the current bit is partitioned into multiple time intervals, and the reference waveform and the data waveform are encoded in a selected one of the time intervals that depends on the current bit.
• Figure IA is a timing diagram of prior art modulation techniques
• Figure IB is a timing diagram of prior art TH-IR modulation
• Figure 2 is a block diagram of a prior art TH-IR receiver
• Figure 3 is a timing diagram of prior art TR-IR modulation
• Figure 4 is a block diagram of a prior art TR-IR receiver
• FIG. 5 is a block diagram of a hybrid-IR transmitter according to the invention.
• Figure 6 is a trellis diagram of Viterbi decoder according to the invention.
• FIG. 7 is a block diagram of a hybrid-IR receiver according to the invention.
• Figure 8 is a diagram of hybrid-IR modulation according to the invention.
• Figure 9 is a two-state trellis diagram for- a differential TR receiver according to the invention.
• Figure 10 is a four-state trellis diagram for a coherent RAKE receiver according to the invention.
• Figure 11 is a block diagram of a hybrid-IR transmitter according to the invention using other modulation formats.
• Figure 12 is block diagram of a hybrid-IR differential receiver according to the invention
• Our invention provides a system and method that enables both TH-IR and TR-IR transceivers to co-exist in the same wireless network.
• Our idea is based on our observation that TR-IR systems encode an information bit as a phase difference between a reference pulse and a data pulse. Furthermore, the polarity of the reference pulse is inconsequential for the correct operation of the TR-IR system.
• TH-IR receiver can decode the information with improved performance, while maintaining the required phase difference or polarity so that a TR-IR can also decode the information.
• This modulation 'hybrid-IR' H-IR
• FIG. 5 show a H-IR transmitter 500 according to the invention.
• the transmitter includes a pre-processor 510 for input bits 501.
• the pre-processor includes a delay 502 and an adder 503.
• the adder sums each input bit 501 to a delayed version of the bit, the sum is inverted 504.
• the pre-processing generates a pair of modulating bits from two successive information bits. It should be noted that more than one pair of modulation bits can be used for each information bit.
• the symbols are modulated 511-512.
• Reference waveforms, e.g., pulses 505, in the sequence are BPSK modulated 511 according to the input bits 501, and data waveforms, e.g., pulses 506, are BSPK modulated according to the inverted sum.
• Waveform generators 521-522 are applied, according to a hopping sequence 530 and delay T d 531 and the results are combined 540.
• the transmitted signal, s ⁇ t) 541, can be expressed as
• the modulation according to equation (4) shows that a phase difference between the reference pulse and data pulse is identical to a conventional TR-IR system.
• Table A shows the four possible combinations of a previous and a current bit, the corresponding values of the reference and data waveforms, and their phase differences or polarities.
• the phase difference between the reference pulse and the data pulse is always 180° regardless of the value of the previous bit. If the current bit is 1, then the phase difference is 0°.
• a TR-IR receiver can demodulate the signal according to the invention.
• the signal can also be demodulated by a TH-IR receiver with improved performance.
• the gain in performance is based on the fact that information is encoded in both the reference pulses and the data pulses.
• the TH-IR receiver can use the energy in the reference pulses to make decisions on the values of the transmitted bits, see Table A.
• a sequence of Nf /2 pairs is transmitted.
• the transmitted signal can be described as follows.
• the transmitter transmits a sequence of Nf 12 pairs.
• the four possible pairs are given by equation (7).
• the pairs are optionally time hopped and scrambled with a polarity code.
• the invention provides a modulation format with 'memory'.
• memory we mean that the encoding of each bit includes information about previously encoded bits.
• Modulation formats that have memory can be represented by a trellis diagram, and decoded accordingly with a Viterbi decoder.
• the transmitted signal is now a two-dimensional signal because two basis signals ⁇ 0 (O and ⁇ ⁇ (t) are used to represent the pair of signals.
• Figure 6 shows a diagram 600 for a Viterbi decoder using a trellis.
• TH-IR receiver can be used to demodulate the signal.
• Our TH-IR receiver is adapted to accommodate the two-dimensional description of the symbol waveform and the memory between consecutive symbols according to the invention.
• FIG. 7 shows the TH-IR receiver 700 according to the invention.
• the RAKE fingers correlate the incoming signal with sequences of the two basis pulses, ⁇ o (t) and ⁇ ⁇ (t).
• the output of each finger is now a 2 -D vector 701.
• the outputs of the finger are summed 710 to produce a soft input observations 702 for a conventional maximum likelihood sequence detector (MLSD) 720.
• the MLSD detector determines a most probable path through the trellis 600 for a given sequence of observations 702. Methods that approximate the MSLD detector, such as Viterbi decoding can also be used.
• Figure 8 shows the relationship between symbols, bits and modulated waveforms.
• the six symbols of the sequence 801 to be modulated are labeled bo to bs, with a previous encoded symbol '0'.
• the symbols in the example sequence are ⁇ 0, 1, 1, 0, 0, 1) 802, which correspond to reference bits ⁇ -1, -1, +1, +1, -1, -1 ⁇ 803, and data bits
• the waveform 805 has the properties described earlier. Specifically, the phase difference between the reference pulse and the data pulse in each pair 806 contains the information about the current bit being transmitted. For each pair the phase difference is 180° when a '0' bit is transmitted, and a 0° phase difference when a '1' bit is transmitted.
• the sequence of pairs also contains the information about the previous bit in the polarity of the reference pulse. Again, this is seen in Figure 8, where the reference pulse in each pair has a +/- polarity that indicates the value of the previously encoded bit. That is, a positive polarity if the previous bit was a T, and a negative polarity when the previous bit was a '0'. It should be understood, that the polarities can all be reversed to achieve the same result.
• This waveform therefore, enables the use of both coherent and differentially coherent receivers, as depicted in Figures 4 and 7 respectively, in the same network.
• the choice of receiver can be based on considerations such as required performance, cost of implementation, or desired transmission distance.
• Generalization, to the case when multiple pairs are used to transmit a symbol, is straightforward. In this case each pair is repeated a number of times, and a polarity scrambling code can be used to improve the spectral characteristics of the waveform.
• a PPM signal may be received using a noncoherent energy detector.
• the additional partitioning of the symbol period into multiple intervals allows the transmitter to modulate bits via PPM as well as the H-IR technique described above.
• a receiver may be used that is based on energy collection or a differentially coherent type receiver, as well as a coherent RAKE receiver.
• the performance of these receivers vary with the more complex architectures achieving better overall bit error rate (BER) performance.
• BER bit error rate
• the addition of PPM modulation also increases the 'memory' of the modulation format and requires that the trellis describing the signal as seen by the differentially coherent receivers and a coherent receiver is modified as is described below.
• BPPM binary PPM
• F first half
• S second half
• the current bit of our bit stream is used to select between one of two possible positions. That is a bit ' 1 ' is encoded in the first interval and a bit '0' is encoded in the second interval.
• the waveform that is transmitted is constructed as described for the H-IR scheme above. Because the current bit is being used to modulate the position of the waveform in this case, the two immediate previous bits are used to modulate the reference pulse and data pulse that constitute the doublets of the symbol waveform. Thus, a simple non-coherent receiver can simply decode the selected transmission interval, i.e., the pulse position. Moreover, we can still use a differentially coherent or coherent RAKE receiver and the higher level trellis encoding/decoding can improve performance.
• mapping of previous bits can be employed to modulate the polarity of the reference pulse, and a proper phase relation with the data pulse is preserved. Additionally, it is noted that this scheme can be further generalized by the addition of PPM modulation on the multi-pulse waveform.
• the length of the memory is two bits, i.e., the immediate previous encoded bits before the current bit bj. That is, bits bi- 2 and bj.i are used to modulate polarity of the reference pulse according to the H-IR scheme above and the bit b ⁇ determines the phase difference and the polarity of the reference pulse while the current bit bj determines the waveform position within the symbol duration. Trellis modulation can be then performed as described below.
• Figure 11 shows changes made to the H-IR transmitter 500 of Figure 5.
• the pre-processor 510 for input bits is modified to embody the addition of additional modulation format.
• the two input bits 501 to the adder are the two previous bits because of the addition of two delay units 1110 and 1111.
• the sum of the two previous bits is inverted 504.
• the current bit 501 now is encoded by another modulation format 1120, e.g., BPPM, to achieve higher orders.
• Another modulation format 1120 e.g., BPPM
• Table B shows eight possible combinations of a current bit and two previous bits, the corresponding values of the reference and data waveforms, and their phase differences or polarities.
• the signal can be demodulated using a noncoherent BPPM receiver that selects the time interval (first half or second half) with the largest receiver energy.
• the signal can also be demodulated by a differentially TR or coherent RAKE receiver with improved performance.
• the gain in performance is based on the fact that information of previously bits, i.e., memory, is encoded in both the reference pulses and the data pulses of the current bit. The additional information can help the TR or RAKE receiver to make decisions on the values of the transmitted bits, see Table A.
• the waveform position (first half or second half) represents the current received bit
• phase difference between the reference and data pulses represents the previously received bit
• Figure 9 shows a two-state trellis 900 decoder that can be used for the decoding.
• a state '0' 910 maps to a previous bit '0'
• a state '1 ' 920 maps to a previous bit '1'.
• Branches 930 of the trellis indicate possible state transitions.
• the branches are labeled with the value of current bit, and a vector representation of the transmitted pair, where the previous bit is demodulated by the phase difference between reference and data pulses, and the current bit is demodulated by the waveform position, and F and S represent first half and second half, respectively.
• Figure 12 shows a TR receiver 1200 according to the invention.
• the receiver After pre- filtering the received signal with a matched filter (MF) 1210 matched to the transmitted waveform, the receiver essentially correlates the received signal 1260 with a delayed version 1220.
• MF matched filter
• the decision is not made after integration 1230 and dump 1270. Instead, a MLSD detector 1240 observes the output of the correlator at the two possible waveform positions and the relative phase difference between pulses from inputs, and determines a most probable path through the trellis 900 based on those observations.
• a decoder 1250 follows. Methods that approximate the MSLD detector, such as Viterbi decoder, can also be used.
• Figure 10 shows a four-state trellis for a coherent RAKE receiver according to the invention.
• a state OO' 1010 maps to previous bits 00
• a state Ol' 1020 maps to previous bits Ol '
• a state '10' 1030 maps to previous bits ' 10'
• a state '11' 1040 maps to previous bits ' 11 '.
• Branches 1050 of the trellis indicate possible transitions. The branches are labeled with the waveform position of current bit, and the vector representation of the transmitted pulse pair.
• the trellis demodulation can be incorporated into the MLSD detector 720 of the RAKE receiver 700 of Figure 7.
• the MLSD detector 720 determines a most probable path through the trellis 1000 for a given sequence of observations. Methods that approximate the MSLD detector, such as Viterbi decoder, can also be used.
• the modulation format according to the invention can be demodulated by- coherent, RAKE TH-IR and a differentially coherent TR-IR receiver.
• the TH-IR receiver according to the invention has improved performance over prior art TH-IR receivers because information is also encoded in reference waveforms. Additionally, by dividing the symbol interval into several intervals and transmitting the waveform in a single interval a simple noncoherent
• the example signals are for a UWB system, it should be understood that the invention can also be used for narrow band width wireless communication systems, and UWB systems that use waveforms other than pulses, CDMA, FSK, and PSK modulation.

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